Here
Brian Kirby, N4FMN describes the modifications that allow
TAC32 software to monitor the Motorola Oncore GPS engine
used in the HP Z3801A. A side benefit of using the GPS
engine signals is a useful front panel one PPS indicator
that does not require hardware pulse stretching.

This
project was designed to allow monitoring of the Motorola Oncore
VP receiver that is used in the HP Z3801A GPS Disciplined Oscillator.
I have experience with the Motorola Oncore VP GPS receivers
and I wanted to monitor the GPS jitter on the 1 PPS signal,
to see how much reduction was performed by the HP Smart Clock
Technology. I also wanted to know about the GPS "sentences"
the HP Z3801A uses.

What
is TAC-32?

The
program I use to monitor the receiver is TAC-32
by CNS. You can download an evaluation copy from them that
will work for 30 days. If you are a member of TAPR, you can
obtain the program from them at the discounted member rate.
There are other programs that can also decode the Motorola binary
strings, such as WinOncore and OutlookGPS.

TAC-32 can decode the Motorola binary signals to ASCII. This
allows you to examine the results. Note that in this implementation,
TAC-32 will only be operated in a receive only mode and cannot
control the GPS receiver.

The circuit uses a Maxim MAX232 integrated circuit for signal/level
translation. The Motorola Oncore VP receiver uses TTL signal
levels. In order to use this data with a computer, the data
must be converted to RS-232. Use shielded mini-coax for the
data and 1 PPS interconnects. In my first attempt, I used unshielded
wire, and I had the MAX232 circuit board installed between the
Z3801A power supply and the main board. I had intermittent results
and I believe that the power supply switching regulators induce
noise into the MAX232 circuit. My wiring connections were on
the top of the GPS receiver.

In this final design, I use RG-174 coax for the receive data
and 1 PPS connections. It is connected to the underside of the
main board and held securely using hot melt glue. I mounted
the MAX232 circuit board near the front of the Z3801A between
the front panel LED board and the 10811 oscillator. I mounted
a DB-9F connector on the rear panel that connects the MAX232
circuit to a computer.

The second part of the circuit allows for a GPS 1 PPS indicator,
which is missing from the Z3801A. I cut the traces to one of
the front panel LED indicators and wired the MAX232 circuit
to the LED. I choose the second LED, labeled "enable" as we
have not found any use for it, and it is not used in the GPSCon
software. I also added another 1 PPS output, which is a
TTL level from the smart clock circuitry in the Z3801A. I mounted
a BNC connector to the rear of the chassis. This pulse is very
narrow, typically 20 to 50 microseconds.

Some
folks are concerned that connections to the GPS receiver may
cause degradation of the Smart Clock function. There is a concern
and the point is well taken. I believe that satisfactory results
may be obtained if shielded interconnects are used and if you
use techniques that will not change the characteristics of the
1PPS circuits within the smart clock.

The Motorola Oncore VP manual shows that the receiver uses inverted
TTL logic. Maxim does not specify the input impedance of the
TTL to RS232 transmitter, but it appears to be a high impedance,
so loading of the 1 PPS circuits should be minimal. The MAX232
transmitter is TTL and CMOS compatible. You can download
a data sheet on the MAX232 from Maxim if you desire to examine
the circuit.

Since modification of the receiver, it has ran a couple of weeks,
and I have not seen any degradation of the smart clock function.
I have two Z3801A's and I have compared it to the unmodified
unit and against a Efratom FRK rubidium oscillator.

This
is the top side of the main pc board of the Z3801A. The
area circled is the +5 volts test point.

This
is a close up of this five volt and ground area. The left
test point labeled "Test point 1 - VCC" is the positive
5 volt power connection for this project. "Test point
2 - Ground" is the common/ground for this project.

This
is the bottom side of the main PC board. These connections
will be the tap point for the Motorola Oncore GPS receiver
signals.

This
is a close up of the GPS connector area. This is the pin
identification for the GPS engine as defined in the Motorola
Oncore reference manual.

This
is a front view of the LED status board. Note the traces
cut for the second from left LED.

This
is the rear of the LED status board. The red wire is the
positive side of the LED, the white wire is the ground
side of the LED.

This
shows the added TTL 1 PPS modification, which goes to
an added chassis mounted BNC connection. Note the connection
is to pin 7 of the IC. This is an optional hardware connection
and is not needed for the MAX232 circuit. Its listed for
reference only, and is not required for the visual one
PPS display or for the TAC32 software interface. This
adds hardware buffering and level translation of the narrow
width one PPS output signal available on the rear connector
of the Z3801A receiver. This narrow width signal is not
suitable for LED indicator viewing without additional
hardware pulse stretching, and is used for optional external
hardware only.